To determine whether temporary cessation of captopril therapy compromises cardiac performance, haemodynamic, hormonal, and electrolyte indices were measured for 2 days before, and 4 days after discontinuation of long-term captopril treatment in 5 patients with heart failure. Captopril withdrawal resulted in a four-fold rise in plasma angiotensin II, higher levels of noradrenaline, and a 13.5mmHg increase in mean arterial pressure. Despite there changes, cardiac output at rest and during exercise was well maintained, and right-heart pressures were unaltered. Although plasma aldosterone levels increased three-fold, neither sodium retention nor potassium depletion occurred. Cortisol levels rose in parallel with angiotensin II levels. These results indicate that in the short term cardiac performance is not impaired and electrolyte balance is not adversely affected by the abrupt withdrawal of captopril.